1. Field of the Invention
The present invention relates to an improvement in an electrode for a non-aqueous electrolyte secondary battery, which can be embodied in either or both of an anode and cathode of such a battery. More particularly, it is concerned with an improvement in the electrode which includes an electrode active material being capable of reversibly intercalating and deintercalating lithium.
2. Description of the Related Art
Secondary battery using a non-aqueous electrolyte and an anode of lithium has a high electromotive Force and is expected to have a higher energy density i.e., a larger capacity per unit weight and volume, as compared with the conventional nickel-cadmium battery and lead-acid battery. Thus, a number of researches and developments have been made on this subject.
Anodes made of metal lithium however have a disadvantage of being liable to develop a dendrite during a charging process, and the developed dendrite causes a short-circuit in the battery, and hence they give only a battery of poor reliability.
In order to overcome this disadvantage, an investigation has been made on an alloy anode comprising lithium and aluminum or lead for the battery. In a battery configured with this alloy anode, lithium is absorbed into the alloy of the anode during the charging process and thus is free from the dendrite formation, and therefore, the alloy anode gives a battery of high reliability. A discharge potential of the alloy anode is higher by about 0.5 V than that of the metal lithium, and hence the voltage of the battery decreases by 0.5 V. Accordingly, the energy density of the battery using the alloy anode is lowered.
On the other hand, research and development have actively been made on a subject of employing, as an anode active material, an inter-layer compound of a carbon material such as graphite and lithium. In an anode comprising the inter-layer compound, lithium enters into a space between the layers of carbon material during the charging process and hence no dendrite is formed. The discharge potential of this anode is higher only by 0.1 V than that of the metal lithium, and a decrease of the battery voltage is accordingly small. These fact ensured that the inter layer compound is more preferable active material for the anode.
The carbon material is generally obtained by heating an organic substance under an inert atmosphere at 400.degree.-3000.degree. C. to decompose the organic substance for its carbonization and graphitization.
In most cases, a starting material for the carbon material is an organic substance, and only the carbon atoms in the organic substance are retained after heating the organic substance up to a temperature at 1500.degree. C., and a graphite structure is grown by heating the remained carbon atoms up to a higher temperature near 3000.degree. C.
As the starting material or the organic substance used in the prior art are a liquid phase material of pitch, coal tar or a mixture of coke with pitch, and a solid phase material of wood material, a fran resin, cellulose, polyacrylonitrile and rayon. Further, a gas phase material is exemplified as gaseous hydrocarbons of methane, propane and the like.
Hitherto, there has been an attempt of employing a so-called graphitizing carbon material as the material for the anode in the non-aqueous electrolyte secondary battery. The graphitizing carbon material may be obtained in general by baking a starting material such as petroleum pitch at a high temperature of 2000.degree. C. or higher and has a grown graphite structure. Another attempt hitherto made is directed to a so-called non-graphitizing carbon material, which is obtained by baking a starting material of a thermo-setting resin exemplified as fran resin at a relatively low temperature of 2000.degree. C. or lower and has a turbostratic structure. Both the graphitizing carbon material and the non-graphitizing carbon material are proposed to be used as the material for the anode capable of intercalating and deintercalating lithium in the non-aqueous electrolyte secondary battery.
On the other hand, MnO.sub.2 and TiS.sub.2 have been enthusiastically investigated as the active material for the cathode. MnO.sub.2 and TiS.sub.2 have a potential with reference to Li of about 3 V. Recently, LiMn.sub.2 O.sub.4, LiNiO.sub.2 and LiCoO.sub.2 have been attracting an attention of this art as the active material for the cathode which exhibits a charge/discharge potential of about 4 V with reference to lithium, and research and development on this active material have been very active. Some of them have already been put in practical use at present.
As mentioned above, there has been and is now continuing a number of efforts directed for increasing a battery voltage as a means for obtaining a high energy density in the battery together with another effort directed for enlarging discharge or service capacity of the battery.
There is also a disadvantage in the above-mentioned case of employing an inter-layer compound of a carbon material such as graphite and lithium as an anode active material. That is, a decrease in the discharge or service capacity occurs in a battery employing this anode active material with a long lasting repetition of charging and discharging cycles.
In order to cope with this disadvantage, a countermeasure is taken by incorporating a fibrous graphite or a glass fiber coated with a carbon material into the anode. These fibrous materials are manufactured by spinning a precursor of carbon fiber or a glass material in general and have a relatively large fibrous diameter of about 6 .mu.m or more and hence are bulky in certain extent. Therefore, in case of incorporating these fibrous materials into the anode, an increased amount of binding agent is required for increasing a strength of an electrode plate, thereby causing another disadvantage of decrease in the initial capacity and the like. In an alternative case of limiting the amount of the binding agent for securing the target initial capacity, the electrode plate becomes to have an insufficient strength, which results in a battery of insufficient charging and discharging cycle characteristics.
A similar disadvantage is also encountered with the cathode. That is, in the cathode, one disadvantage to be overcome is to effectively prevent a decrease in the discharge or service capacity with a long lasting repetition of charging and discharging cycles. In order to overcome the disadvantage, a number of efforts have been made on an improvement in the cathode active material, on an investigation of the electrolyte, on an improvement in a separator, and the like.
There are many causes for the decrease in the discharge or service capacity of a battery in case of repetitive charging and discharging cycles. One of the causes is a phenomenon that the cathode active material is reduced to its minuteness by the repetition of charging and discharging cycles even in the above-mentioned compound capable of intercalating and deintercalating lithium. The phenomenon of reducing the active material to minuteness becomes remarkable with a continued repetition of charging and discharging cycles. As a result, the electrode is finally disintegrated in a case wherein the above-mentioned phenomenon has been most proceeded. Therefore, many investigation have also been made on the binding agent and the result of some investigation are proposed for coping with the above-mentioned phenomenon. At present, a fluorocarbon resin, a rubber resin, a polyolefine or the like is preferably employed as the binding agent.
Even with the employment of an investigated and proven binding agent, there is still a disadvantage that a sufficient cycle characteristics cannot be obtained with the battery incorporating the binding agent. It is believed that the cathode active material expands and contracts with the intercalation and deintercalation of lithium, and this causes a defect in the electrode ability of holding the active material and another defect of poor collecting of current.
In order to overcome this disadvantage, a countermeasure has been taken to incorporate a fibrous graphite or a glass fiber coated with a carbon material into the cathode as in the case of the anode. As previously described, these fibrous materials are generally bulky. Therefore, as in the case of the anode, an increased amount of binding agent is required for increasing the strength of an electrode plate, thereby causing another disadvantage of decrease in the initial capacity and the like.
In an alternative case of limiting the amount of the binding agent for maintaining the target initial capacity, the electrode plate becomes to have an insufficient strength, resulting in a battery of insufficient charging and discharging cycle characteristics.